Because plastic materials are typically light, have considerable toughness and are easily dyed, they have come to be used in lenses and various other optical applications in recent years. However, in the case of using plastic materials in applications such as eyeglass lenses, if the refractive index is low, the thickness of the corresponding lenses increases considerably with an increase in the desired diopter value. As this also increases the weight, there is the problem of it being difficult to take advantage of the characteristic of plastic materials of being light. In addition, as plastic materials also have the characteristic of the Abbe number becoming lower the higher the refractive index, which tends to increase color aberration, there is the problem that this large color aberration causes viewed objects to become colored and appear blurred. In consideration of these problems, there is a need to develop a material for use as an optical plastic material having both a high refractive index and high Abbe number.
Examples of such optical plastic materials include a polythiourethane obtained by reacting a polythiol and polyisocyanate as disclosed in Japanese Examined Patent Publication No. H4-58489, and the use of a polythiol component of a polythiourethane having a high sulfur content as disclosed in Japanese Unexamined Patent Publication No. H5-148340. In addition, Japanese Unexamined Patent Publication No. 2001-11182 discloses a polythiourethane obtained by ring-opening polymerization of a cyclic thiocarbamate compound. Since these polythiourethanes of the prior art generally have a high refractive index of 1.6 or more, and a high Abbe number of about 30, they are frequently used in thin, lightweight eyeglass lenses.
However, polythiourethane had problems which could not be ignored in terms of practical use such as the strength with respect to a load such as tension, bending or impact (dynamic performance) becoming lower the higher the refractive index thereof. For example, as eyeglass lenses and the like are required to have optical performance in terms of high refractive index and high Abbe number as previously described, as well as dynamic performance in terms of superior tensile characteristics and high elasticity (high elongation percentage, high shape recovery), there has been a desire to develop a plastic material capable of satisfying both of these types of performance.
On the other hand, thiocarbonate plastic materials have been proposed as optical plastic materials having both a high refractive index and high Abbe number. For example, Japanese Unexamined Patent Publication No. H6-116337 discloses a polymer of a thiocarbonate compound having a vinylphenyl group on a terminal thereof and a polythiol compound, while Japanese Unexamined Patent Publication No. H11-228697 discloses a thiocarbonate sulfur-containing polymer obtained by ring-opening polymerization of a cyclic thiocarbonate compound. However, the former has problems such as the production process itself of the raw material thiocarbonate being complex, the refractive index of the resulting polymer being only slightly greater than 1.6, and the refractive index of the polythiourethane not being improved, while the latter has problems such as the specific physical properties of the resulting sulfur-containing polymer being unknown and the types of resulting sulfur-containing polymers being limited because the raw material is limited to specific cyclic compounds (namely, specific cyclic compounds having a small number of carbon atoms).
In addition, Japanese Unexamined Patent Publication No. H11-292969 discloses a thiocarbonate polymer obtained by ring-opening polymerization of a heterocyclic compound obtained by reacting carbon disulfide with an epoxy compound (such as propylene oxide), a thiocarbonate polymer obtained by ring-opening polymerization of a heterocyclic compound obtained by reacting a carbonic acid ester with a compound having a hydroxyl group and thiol group in a molecule thereof, and a thiocarbonate polymer obtained by reacting the aforementioned heterocyclic compound with a poly active hydrogen compound, as examples of thiocarbonate plastic materials having a high refractive index and superior mechanical strength. However, as all of these polymers use a heterocyclic compound for the raw material, there are restrictions on the type of raw material, thereby resulting in the problem of limitations on the types of polymers which can be obtained, as well. In addition, Japanese Unexamined Patent Publication No. H11-292969 only specifically discloses the initial open-ring polymer of the heterocyclic compound, while the physical properties of other polymers are unknown.
Moreover, a polythiocarbonate obtained by reacting phosgene and an alkane dithiol is disclosed as a plastic material having a high refractive index in Polymer, 35, 7, 1564 (1994) and Polymer Communications, 1990, 31, 431. However, this polythiocarbonate was not suitable as a raw material for obtaining polythiourethane having superior optical and dynamic performance since the polythiocarbonate is produced by using phosgene which is highly toxic, as a raw material, has a high molecular weight and is provided with chlorine atoms on the terminals thereof.
It could therefore be helpful to provide a polythiourethane provided with the performance required of an optical plastic material to be for eyeglass lenses and similar applications, and a particularly superior optical performance (high refractive index and high Abbe number), as well as a superior dynamic performance such as superior tensile characteristics and high elasticity (high elongation percentage and shape recovery).